Wear OS watches benefit from prompt updates provided through the Wear OS app.
When an update is available, you’ll get notified with a watch card.

Wear OS updates enable even more features, but rely on the watch OEM getting involved

Wear OS updates can also add new features, and come quickly because they’re just through the watch Play store, the OEM does not need to be involved.

Versions

Wear OS and Android Wear versions had numerical overlap, so don’t be confused by old Android Wear pages.
The “System Version” and “Wear OS” versions are not related.

Wear OS System H

Wear OS System H has been
rolling out
to contemporary Wear OS devices since November 2018.
Since
Wear OS H
is an upgrade to Android 9, it requires OEM integration.
Unlike the typical Wear OS updates that come to most Wear OS devices quickly, Wear OS H will come to only some watches, most likely new / newer watches.

Enhancements

connection status display, with up/down activity arrows on whichever connection method is being used (LTE, WiFi, or Bluetooth/BLE).

auto-size text, based on how much text is in the notification

better rejection of accidental side-swipe and long-press

Switchable Bluetooth

Wear OS allows easy toggling of the watch Bluetooth and Bluetooth LE (BLE).
This means the watch doesn’t search for the phone over BLE, saving battery.
This could be useful for users that leave their phone deliberately behind for jogs and the like.
Most users won’t use this, but for geeky/power users that like to experiment with Wear OS wireless, it’s really nice to have manual Bluetooth enable/disable.

Find my Phone–over LTE

If you have a large home or office where you could be out of Bluetooth LE range, with Wear OS you can use Find My Phone to make your phone ring over LTE, Wifi, and Bluetooth.

Note that “find my device” to find the watch only works when the watch has LTE.
Remotely locating the watch does not appear to work when the watch is on 2G or 3G.

Blackberry Hub+

By tapping on an email notification in Wear OS, you get the first several lines of the email.
You can also reply to emails from the smartwatch.

Uber standalone

Assisted by
GPS / LTE based Wear OS location,
the standalone Uber app is one of a growing fleet of standalone Wear OS apps that work great from the watch, even if you forgot your phone or the battery died.
I believe this Uber standalone app will save lives and improve safety for those with a dead phone who otherwise might be stranded and have to find a riskier way home–instead they can use Uber from their Wear OS watch.

Wear OS security

auto-lock: Wear OS locks within a second or so of the watch being removed from wrist.

Particularly if your institution has a single signon (SSO), you could be making a major security risk–don’t enter such a password into a third-party app.

Smartwatch encryption

Android Device Policy, a BYoD-focused app, does not allow your Wear OS smartwatch to use your work email as the primary account if your administrator requires encryption.
Wear OS does not use device encryption.

Workaround: Use a personal gmail.com address as the primary device address.

Login security

Wear OS allows PIN, Password, or Swipe pattern.
Wear OS for better security should force you to unlock your phone with password and then reauthorize the watch after say 3 or 5 failed attempts to unlock watch.
Right now after 3 failed attempts with Wear OS, you get locked out for 1 minute.